A) Field of the Invention
The present invention relates to a manufacture method for a ZnO-containing compound semiconductor layer.
B) Description of the Related Art
Zinc oxide (ZnO) is direct transition type semiconductor having a band gap of about 3.3 eV at a room temperature and an exciton binding energy of 60 meV higher than that of other semiconductors, and is expected as the material of a high efficiency light emitting device. Mixed crystal such as MgxZn1-xO is also used to adjust a band gap. ZnO and mixed crystal having the same crystalline structure as that of ZnO are herein called ZnO-containing compound semiconductor.
In manufacturing, for example, a ZnO-containing compound semiconductor light emitting device, it is very important to control epitaxial growth of a thin film at an atomic level in order to improve the function of the light emitting device. One of recent leading research approaches to epitaxial growth of a thin film is surfactant mediated epitaxy. This approach artificially changes a growth mode of a thin film by using surface active agent (atoms or molecules) called surfactant, and is a useful means for epitaxial growth control.
JP-A-2004-221352 (which is incorporated herein by reference) discloses techniques of using hydrogen as surfactant when a ZnO-containing compound semiconductor layer is grown.
JP-A-2007-128936 (US-2007-0134842, which is incorporated therein by reference) discloses a temperature range of two-dimensional growth when an undoped ZnO layer is formed on a Zn polar surface of a ZnO substrate. FIGS. 1A to 1D in this Publication are reproduced in FIGS. 11A to 11D of this application. FIG. 11A shows a relation between a substrate temperature and a growth rate. FIGS. 11B to 11D show RHEED images of ZnO layers grown at substrate temperatures of 800° C., 850° C. and 1000° C., respectively (RHEED images of ZnO layers corresponding to plots 1B, 1C and 1D in FIG. 11A). At a substrate temperature not lower than 850° C., the RHEED images indicate streak patterns and it can be known that two-dimensional growth occur (FIGS. 11C and 11D). At a substrate temperature lower than 850° C., the RHEED image indicates spot patterns and three-dimensional growth occurs (FIG. 11A).
This Publication further discloses the following knowledge regarding a relation between a ratio between Zn and O and a growth mode. A flux intensity of Zn is represented by JZn, and a flux intensity of O radical is represented by JO. A coefficient (Zn sticking coefficient) indicating a bonding feasibility of Zn to an O terminated plane of ZnO crystal is represented by KZn, and a coefficient (O sticking coefficient) indicating a bonding feasibility of O to a Zn terminated plane is represented by KO.
A product KZn·JZn of the Zn bonding coefficient KZn and the flux intensity JZn corresponds to the number of Zn atoms bonded in a unit area of a substrate per unit time. A product KO·JO of the O bonding coefficient KO and the flux intensity JO corresponds to the number of O atoms bonded in a unit area of a substrate per unit time. A ratio of the product KO·JO to the product KZn·JZn is defined as a flux ratio. The flux ratio larger than 1 is called an O rich condition, whereas the flux ratio smaller than 1 is called a Zn rich condition.
H. Kato. M, Sano, K, Miyamoto, T. Yao: Journal of Crystal Growth 265 (2004); pp. 375-381 which is incorporated herein by reference, reports that RHEED images of ZnO layers had streak patterns, the ZnO layers being formed under an extremely O rich condition (flux ratio ≧5.6) and at various flux ratios, and suggests a ZnO layer is likely to grow three-dimensionally if the O rich condition is not extreme.
For example, a ZnO-containing compound semiconductor light emitting device has a multi-layer structure laminating an n-type semiconductor layers an emission layer a p-type semiconductor layer and the like. Improving flatness of an interface between lower layers is desired to form upper layers by good epitaxial growth. Techniques of improving flatness of a ZnO-containing compound semiconductor layer has been long desired.